In the interpretation of the features shown in a vertical view of a mountainous country the orientation of the photograph is of prime importance. When viewed in proper orientation, that is, as already pointed out (p. 5), with the shadows falling toward the observer, mountains and valleys appear in their correct relation. But, if the position of the picture is reversed, a mountain will look like a depression and a valley like a ridge. This reversal of the image can be tested by looking at Figures 49 or 52 from both viewpoints. However, since the vertical photographs will be compared with maps of the same area, it is thought better to place them on the page as if they were maps. In order to make them appear natural the prints can be turned in the necessary direction.
Fig. 45—Map of the Yosemite Valley, showing the area included within the angle of vision of Fig. 44. The map, a reduced section from the Yosemite and Mt. Lyell, Cal., topographic sheets, 1:125,000, published by the U. S. Geological Survey, is oriented for direct comparison with the photograph. Scale, 1:167,000.
Fig. 46—Mountains of volcanic origin: Cinder Cone with, in the distance at the right, Lassen Peak in the northern Sierra Nevada, California, as seen from an airplane over Lake Bidwell. Beyond the lake appears the rough surface of lava poured out as molten rock less than two hundred years ago (see U. S. Geol. Survey Bull. 79, 1891). Surrounding the cone is a light-colored ash field, sparsely forested at the right, which was formed about two hundred years ago. The mountain in the middle of the photograph having a smooth surface is Cinder Cone, rising 640 feet above the general level of the ash field and consisting of fragments of lava—the so-called ash and cinders—blown from the crater at times of eruption.